Downy mildew has reared its ugly head on basil on the US eastern seaboard. First appearing in Florida in 2007, it has since moved up the coast and is now reported in Georgia right up to Pennsylvania and west to Indiana and Illinois skipping over to North Dakota and on to California.
Ocimum basilicum appears most susceptible to mildew (the most commonly grown home variety)
The cultivars (mostly commercial ones tested) ‘Martina’, ‘Nufar’, and ‘Poppy Joe’s’ were highly susceptible.
Ocimum × citriodorum and O. americanum were less susceptible.
Hybrids ‘Spice’, ‘Blue Spice’, and ‘Blue Spice Fil’ showed no mildew symptoms
Top of leaf starts to yellow. But the fungus grows under the leaves so you have to flip the leaf over to see.
The fungus is gray to purple-gray and appears to be dusty (sometimes turning to black when fully mature and spreading).
Remove all healthy leaves – cut them off
Cover plant with plastic bag and tie bottom around stem at soil line. Cut stem below bag. We cover it with a bag so the spores are not spread into the garden.
Remove from garden – do not compost.
The problem overwinters in the soil so crop rotation is a must.
Organic controls involve crop rotation, spacing, watering in the early morning to avoid wet leaves all day/night and if all fails, the use of a copper based fungicide such as bordeaux mixture or some of the soap/copper combinations.